1. Field of the Invention
The present invention relates to a method and an apparatus for compressing image data, a method and an apparatus for decompressing image data that have been compressed, and an image display method and an image display apparatus for displaying decompressed image data. The present invention also relates to a program to cause a computer to execute the image data compression method, the image data decompression method and the image display method.
2. Description of the Related Art
A system for obtaining image data by reading a radiation image of a subject recorded on a stimulable phosphor sheet or on a film and for reproducing the image by using a display apparatus or the like after carrying out appropriate image processing thereon has been proposed. In such a system, image data obtained in the above manner are recorded in a recording medium such as an MO disc after being compressed to reduce an amount of data. As file formats for storing image data by compression, various formats such as JPEG, GIF and TIFF are used.
In the file formats for image data compression described above, two types of compression method, namely reversible compression and irreversible compression, have been defined. In reversible compression, original image data can be restored when image data compressed according this method are decompressed. In irreversible compression, some of information is lost in a process of compression, and original image data cannot be restored completely. In irreversible compression, the larger a compression ratio is, the more image quality degrades after decompression of compressed image data. Therefore, the compression ratio is set in consideration of image-quality degradation. Upon compression, compression information regarding compression processing such as the compression ratio and the compression format is described as tag information in a file header of compressed image data. The compressed image data can be decompressed with reference to this compression information.
After image data obtained by decompressing image data that have been compressed in the above manner are reproduced, irreversible compression is then carried out in some cases on the decompressed image data when the decompressed image data are stored. Since a portion of information of the original image data is lost in the irreversible compression, the quality of an image obtained by the compression degrades. Therefore, the image quality degrades repeatedly if the image data obtained by decompressing the irreversibly compressed image data are subjected to repetitive irreversible compression. Meanwhile, the compression information is added to the compressed image data. The compression information is overwritten upon compression in order to indicate the latest compression processing. Therefore, if the image data obtained by decompression of the image data having been subjected to repetitive irreversible compression are reversibly compressed in order not to degrade the image quality, the image quality becomes lower than that of an original image, regardless of the compression information indicating reversible compression. Therefore, the compression information contradicts the image quality. Especially, in the case of radiation image data, if a diagnosis is carried out on the assumption that an image used therefor has high quality, a serious misdiagnosis such as overlooking focus may occur in the case where the image is actually not high-quality.